Many communication signals are composed of a carrier signal, often operating at an RF or microwave carrier frequency, which is modulated with a baseband signal which conveys the real information in the communication signal. One well known example of such a communication signal is an amplitude modulated (AM) signal, wherein the baseband signal modulates the amplitude of the carrier signal.
FIG. 1A illustrates an example embodiment of an apparatus 100 for producing an AM signal. Apparatus 100 includes a carrier signal generator 110, a baseband signal generator 120, and an AM modulator 130.
Carrier signal generator 110 generates a carrier signal 115, in particular a periodic signal (e.g., a sine wave) having a carrier frequency. In various embodiments, carrier signal generator 110 may comprise an oscillator, a frequency synthesizer, or a waveform generator.
Baseband signal generator 120 in general can be any kind of device which generates a baseband signal 125 which can be used to modulate the amplitude of carrier signal 115. In some embodiments, baseband signal 125 may comprise a digital signal, or low-pass-filtered digital signal, in which case baseband signal generator 120 may be any kind of digital signal generator.
AM modulator 130 receives carrier signal 115 and modulated the amplitude of carrier signal 115 with baseband signal 125.
FIG. 1B illustrates another example embodiment of an apparatus 100 for producing an AM signal. In apparatus 150, baseband signal generator 120 provides baseband signal 125 to carrier signal generator 110 and the amplitude modulation is performed directly at carrier signal generator 110 such that no separate AM modulator is required.
FIG. 2 illustrates an example embodiment of an amplitude modulation (AM) signal 135, and FIG. 3 illustrates an example embodiment the frequency spectrum of an AM signal. In FIGS. 2 and 3, baseband signal 125 is a periodic signal having a frequency which is less than the carrier frequency. However, in general baseband signal 125 may be any arbitrary signal. However, in general, the highest frequency component of baseband signal 125 should be less than the carrier frequency of carrier signal 115, and is often much less than carrier frequency (e.g., by one or more orders of magnitude).
At a communication receiver, an AM signal may be demodulated to extract the baseband signal containing the information which is communicated. There are many well-known techniques for extracting the baseband signal by performing frequency domain signal processing of the AM signal. Perhaps the most common AM demodulator consists of a rectifier followed by a low pass filter. The operation of such an AM demodulator may be easily understood with reference to the frequency spectrum. The rectifier effectively squares the AM signal and thereby reproduces copies of the AM signal spectrum around twice the carrier frequency and also around DC or 0 Hertz. The output of the rectifier at twice the AM carrier frequency is removed by the low pass filter, leaving only the portion around 0 Hertz—which has the same spectrum as the original baseband signal.
Often a communication device or system which employs AM, or the AM signal itself, needs to be tested and analyzed. In particular, often it is necessary for a test instrument to extract the baseband signal from an AM signal which is being analyzed. This may include displaying the baseband waveform of the baseband signal to an engineer or technician who is analyzing the AM signal, or evaluating the performance of a device or system which generated or employs the AM signal.
Frequently, a digital oscilloscope is the primary, or only available, test instrument to test and analyze such an AM signal, or the device or system which generates or employs the AM signal.
However, in general, a digital oscilloscope does not include the frequency domain hardware (e.g., a rectifier) to perform efficient frequency domain signal processing of the AM signal to recover the original baseband signal.
Many modern digital oscilloscopes do include powerful mathematics and digital signal processing capabilities. The original baseband signal can be recovered from an AM signal using frequency domain digital signal processing techniques such as Fast Fourier Transforms (FFTs) and the Hilbert transform. Another technique involves tasking the square or absolute value of the AM signal, and passing the resultant signal through a low pass filter to remove the carrier. In the case where the square of the AM signal is employed, the square root of the output of the low pass filter should be obtained.
All of the above-described techniques for processing the AM signal by a digital oscilloscope tend to be computationally intensive and can take a long time to extract the baseband signal. Compounding this is a growing need to analyze long time windows to understand complex system behavior, including for example initialization and handshaking. This can require processing a large amount of data.
Thus it would be desirable to provide a more convenient and more reliable method and system to measure and characterize an electrical system or network which employs amplitude modulation.